The invention relates to an apparatus and a method for determining the proportion of a substance in a gas sample using an infrared measuring instrument.
From European Patent Disclosure EP 584 519 B1, an apparatus and a method for measuring the concentration of a substance in a gas sample with an infrared measuring instrument has been disclosed. A radiation source, triggered by a modulated current, shines through the gas sample to be examined, and the measurement radiation is recorded by a measuring detector and a reference detector. The measurement and reference signals output by the respective detectors are composed of a dark signal, a direct voltage component, and an alternating voltage component. The dark signal is established as a substantially constant offset voltage, when the radiation source is darkened.
The direct voltage component and the alternating voltage component are each dependent on the measured concentration.
The ratio between the alternating voltage component and the direct voltage component is independent of the concentration, and is a function of only the degree of modulation of the radiation source.
The concentration of the substance is determined from the direct voltage component of the measurement signal, and the alternating voltage component of the reference signal is used for standardizing the direct voltage component of the measurement signal.
The known measuring instrument necessitates highly complicated circuitry to split up the measurement signal and the reference signal into the different voltage components. The measurement method is suitable only for detectors that have a defined dark signal, and only short-wave infrared radiation in the absorption range of carbon dioxide can be detected.